MOS transistors with improved withstanding voltage characteristics have recently been developed (for instance, see Japanese Patent Laid-open Publication No. 2005-294584). As an example, an n-type MOS transistor is illustrated below. If a low concentration n-type region is located between the p-type region, which acts as channel region, and the n-type drain region, moderate changes in voltage takes place in the low concentration n-type region, serving to improve the withstanding voltage characteristics. In addition, the use of a field plate has been proposed as a means of reducing the electric field concentration that occurs in the vicinity of the drain-side edge of the gate electrode (for instance, see Japanese Patent Laid-open Publications No. 2006-310580, No. 2001-7327, and No. 2009-302548).
An insulating film is formed on the low concentration n-type region, and another electrode called field plate is formed on this insulating film. Application of voltage to the field plate moderates the changes in the vicinity of the drain-side edge of the gate electrode, which serves to improve the withstanding voltage characteristics of the MOS transistor.
In Japanese Patent Laid-open Publication No. 2006-310580, the gate insulation film is shared by the field plate which uses it as the aforementioned underlying insulation film. With this technique, however, it is difficult to adjust the thickness of the insulation film under the field plate independently of the gate insulation film.
In Japanese Patent Laid-open Publication No. 2001-7327, an insulation film is formed under the field plate by thermal oxidation independently of the gate insulation film. This technique, however, requires a thermal oxidation step to form an underlying insulation film for the field plate.